Moisture in wood decreases the stiffness of the fibre walls and enlarges the region of failure.  The curve which the fibre walls make in the region of failure is more gradual and also more irregular than in dry wood, and the fibres are more likely to be separated.

In examining the lines of rupture in compression parallel to the grain it appears that there does not exist any specific type, that is, one that is characteristic of all woods.  Test blocks taken from different parts of the same log may show very decided differences in the manner of failure, while blocks that are much alike in the size, number, and distribution of the elements of unequal resistance may behave very similarly.  The direction of rupture is, according to Jaccard, not influenced by the distribution of the medullary rays.[7] These are curved with the bundles of fibres to which they are attached.  In any case the failure starts at the weakest points and follows the lines of least resistance.  The plane of failure, as visible on radial surfaces, is horizontal, and on the tangential surface it is diagonal.

[Footnote 7:  This does not correspond exactly with the conclusions of A. Thil, who says ("Constitution anatomique du bois,” pp. 140-141):  “The sides of the medullary rays sometimes produce planes of least resistance varying in size with the height of the rays.  The medullary rays assume a direction more or less parallel to the lumen of the cells on which they border; the latter curve to the right or left to make room for the ray and then close again beyond it.  If the force acts parallel to the axis of growth, the tracheids are more likely to be displaced if the marginal cells of the medullary rays are provided with weak walls that are readily compressed.  This explains why on the radial surface of the test blocks the plane of rupture passes in a direction nearly following a medullary ray, whereas on the tangential surface the direction of the plane of rupture is oblique—­but with an obliquity varying with the species and determined by the pitch of the spirals along which the medullary rays are distributed in the stem.”  See Jaccard, op. cit., pp. 57 et seq.]

SHEARING STRENGTH

Whenever forces act upon a body in such a way that one portion tends to slide upon another adjacent to it the action is called a shear.[8] In wood this shearing action may be (1) along the grain, or (2) across the grain.  A tenon breaking out its mortise is a familiar example of shear along the grain, while the shoving off of the tenon itself would be shear across the grain.  The use of wood for pins or tree-nails involves resistance to shear across the grain.  Another common instance of the latter is where the steel edge of the eye of an axe or hammer tends to cut off the handle.  In Fig. 10 the action of the wooden strut tends to shear off along the grain the portion AB of the wooden tie rod, and it is essential that the length of this portion be great enough to guard against it.  Fig. 11 shows characteristic failures in shear along the grain.